Airlines provide space beneath seats for storing carry-on articles. Usual dimension limitation for such articles is that the sum of the length plus width plus height should not exceed 45 inches.
A loaded bag falling within this dimension criterion can weigh over 40 pounds and become difficult to carry. These bags are therefore sometimes fitted with friction slides, small wheels or ball casters on their bottoms and pulled by means of a single strap or handle fastened to a clip or ring located in the middle of the leading or front section of the bag. To facilitate steering or directing the bag, front caster wheels are swivel-mounted while the rear wheels are rigidly mounted for stability. With friction slides, swivel-mounted front caster wheels and ball casters, the bags have a tendency to swerve back and forth as they are pulled by means of a single strap or handle. Such swerving usually causes the bag to overturn.